The accurate measurement of ambient fluid (air) flow is becoming increasingly more important in the application and control of many processes, as well as in the research laboratory. One of the major applications is in the field of air sampling, where an accurate knowledge of the sampled air quality determines the exposure level to various contaminants. The most widely accepted primary standard method for a gaseous fluid is the bubble flow meter. In the basic form of the bubble flow meter, a soap film is generated from a soap solution, and is propelled by the gas flow under measurement from one end of the flow meter to the other. By timing the rise of the soap film between calibrated volume marks, the volume flow is obtained. Since for all practical purposes, the soap film is massless, it requires almost no force to accelerate the bubble. Furthermore, a seal is always insured by the presence of the bubble. The very nature of the bubble eliminates the friction which is associated with a piston type flow meter. The soap film flow meter is essentially transparent to the flow being measured, having a no-load effect. Accordingly, the soap film flow meter comes closest to meeting the unique requirements of the ideal calibrator.
The measurement of air flow using a positive displacement reciprocating piston flow meter is susceptible to errors based on the following requirements:
1. Initial breakaway friction;
2. Acceleration and deceleration of the piston after breakaway (until equilibrium is reached);
3. Running friction; and
4. Fixed pressure loading determined by the mass of the piston.
The resolution of the above conditions presents a load to the air flow system being measured. The arrangement of the present invention minimizes the initial breakaway friction and acceleration forces of the piston on reversing its direction at the bottom of the piston stroke and, if desired, permits automatic operation without an external power source. An additional advantage of the positive displacement piston flow meter of the present invention is its simplicity in design for reversing the direction of the piston on both the upstroke and downstroke of the piston.